In a modern radio communications system, a single radio transceiver may have the capability of accessing a large number of radio communications channels. In general, one or two of these channels will be designated "priority" channels for the individual using the radio. Important communications intended for this individual, such as emergency calls, will ordinarily occur on this channel.
Since the calls that occur on such a priority channel are, by definition, important, the individual has an obvious interest in hearing them. By the same token, that individual will also wish to be able to communicate on other channels as well. An obvious conflict develops; how to allow the individual the flexibility of monitoring or communicating on a non-priority channel while simultaneously remaining cognizant of pertinent communications on his priority channel(s).
One solution is to provide the individual with more than one radio; i.e., a separate radio for each priority channel, and another for non-priority communications. This solution presents serious cost and convenience obstacles.
Another solution has been to provide channel scanning features. From time to time, the radio will automatically switch to receiving the priority channel and monitor for activity. If activity is detected, the radio unsquelches and allows the operator to hear the priority channel activity. In the absence of activity, the radio returns to the communication in progress. Since channel activity can be detected in only a few milliseconds, such scanning activity can occur relatively transparently with respect to use of other channels by the operator.
The above solution will not adequately support all communications needs, however. In many systems, a number of different groups may use the same channel resources. As a result, communications occurring on a priority channel may not in fact be intended for all radio operators. If a radio in such an operating environment interrupts an ongoing non-priority communication and unsquelches on such a communication occurring on a priority channel, the operator's ability to make satisfactory use of the radio becomes highly impaired.
One solution to the above situation has been to include an identifying signal, such as a continuous tone, with the information signal. Through appropriate filtering, the presence or absence of a particular identifying signal can be detected by the radio. When a particular signal is so identified, the radio will then unsquelch and allow the operator to participate in the communication. Through use of such identifying signals, an operator will be spared the annoyance and disruption of having the radio unsquelch on irrelevant communications.
The above approach gives rise to new problems, however. In particular, since the identifying signals are generally precise frequencies in order to allow provision of a suitable number of signals to support the desired function, the detection filters must be narrowband (typically 3 Hertz or so). As a result, detection of the signal can consume an objectionable period of time, such as 150 to 250 milliseconds. When channel scanning, such a "hole" in an ongoing communication on another channel can be quite noticeable and annoying. To deal with this to some extent, some prior art radios simply remain on the priority channel when activity has been detected, but do not unsquelch in the absence of the correct particular identifying signal. Such an approach simply concedes the functional inoperability of this approach with respect to interrupting an ongoing communication, and makes the best of a bad situation by at least not providing the operator with an irrelevant communication.
A need clearly exists for a channel scanning and identifying tone detection device and method that provides minimal disruption to an ongoing communication while simultaneously assuring that pertinent priority communications are detected and unsquelched in a rapid manner.